As the coastal fronts being detected by the SeaWiFS-derived Chl-a concentrations and AVHRR-derived SST images, a three-dimensional primitive equation numerical model is used to study the physical mechanisms for the formation of fronts around the Hainan Island, northwest of the South China Sea. Several monthly averaged SeaWiFS-derived Chl-a concentrations indicate that higher Chl-a zones usually exist along the coasts of the Zhanjiang Peninsula, the Beibu Gulf (or the Gulf of Tonkin) and the Hainan Island. Obvious surface Chl-a fronts can thus be seen narrowly adjacent to the higher Chl-a zones. AVHRR-derived SST images indicate several other fronts that have a large seasonal variation. The SST fronts are formed in the Beibu Gulf and in the northeastern area off the Hainan Island in winter. The numerical modeling of seven major tidal constituents (M2, S2, K1, O1, P1, N2, and K2) indicates that two high kinetic energy zones appear in the Qiongzhou Strait and near the southwestern coast of the Hainan Island. The value of log (h/u3), where h is depth in meter and u is the depth-mean tidal current in m/s, is calculated to predict the frontal position. It is indicated that the locations of contour lines log (h/u3) = 2.9 ~ 3.0 are in good agreement with the frontal positions detected by the satellite observations. Therefore these Chl-a fronts can be considered to be mainly induced by the tidal mixing while the monsoon wind, solar radiation, and coastal dynamics may play an important role for the seasonal variation of several other fronts.